1. Field of the Invention
The present invention relates to an eccentric rotor used as a silent call device for a mobile communications apparatus, a compact vibrator motor having the rotor and a method of manufacturing the rotor, and more particularly, to an improvement of assembly and structure of an eccentric rotor which does not require an eccentric weight.
2. Description of the Related Art
Referring to FIG. 21, as a silent call device for a pager or a mobile phone, an eccentric weight W made of tungsten alloy is coupled to an output shaft S of a cylindrical DC motor M. When the motor M rotates, vibrations are generated due to the difference in centrifugal force of the eccentric weight W.
However, as the above addition of the eccentric weight W to the output shaft S requires a space for rotation of the eccentric weight W in an apparatus such as a pager, there is a limit in designing the apparatus. Also, use of the expensive tungsten alloy increases the production costs.
The present applicant have suggested a cylindrical coreless vibrator motor in Japanese Patent Application No. Hei 2-309070 and the corresponding U.S. Pat. No. 5,107,155, in which a built-in rotor itself is made eccentric excluding an output shaft. The above motor having no output shaft and no eccentric weight is favorably noticed by the market as there is no limit in design, use thereof is easy and there is no danger during rotation. However, as the motor requires three cylindrical coreless coils, the number of parts or processing steps increases, thus increasing the production costs.
In order to make a rotor with a core itself vibrate instead of the cylindrical coreless coil type, the present applicant has suggested removing one of three salient pole type cores as shown in FIG. 4 of Japanese Patent Publication No. Hei 6-81443. The above two salient pole type cores where one pole in three phases is missing are preferable in the case of a motor such as a massager needing a relatively large amount of output. However, for a portable apparatus such as a portable terminal using a low voltage, as the portable apparatus is small, movement of the center of mass is little and the amount of vibrations is insufficient.
Also, as disclosed in U.S. Pat. No. 5,341,057, the present applicant has suggested a compact vibrator motor having an eccentric armature iron core which is formed by arranging three salient poles made of magnetized material at one angular side with respect to a rotor to face a field magnet having four alternate north and south pole sections. Also, the same technical concept has been disclosed in Japanese Laid-open Patent Application No. 9-261918. However, as the three armature iron cores made of magnetized material are distributed at one angular side and cogging torque (a force of being absorbed by a field magnet) increases in the case of the motor, pores needs to be enlarged and the diameter of the motor itself cannot be reduced.
The above motor having a built-in type eccentric rotor becomes a shaft-fixed type as it does not need an output shaft. As the size of the above motor is reduced, the distance between armature coils decreases. Thus, connection of the end portion thereof to the commutator without damage to the armature coil is very difficult. Particularly, when a printed circuit board is used as a flat panel commutator as it is, where the end portion of the armature coil is directly welded thereon, welding of the end portion is not easy as the end portion is easily detached from a printed pattern due to elasticity of the end portion.